This application claims priority from Japanese Patent Application No. 2002-003136 filed Jan. 10, 2002, which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical pulse pattern generator, and more particularly to an optical pulse pattern generator capable of generating a pulse train for optical labeling and an optical random pattern pulse train for device evaluation in an optical communication field.
2. Description of the Related Art
As optical communication systems increase their capacity, high-speed transmission systems with a bit rate of 40 Gb/s per channel is nearing practical use. In addition, major carriers and vendors in the U.S., Europe and Japan promote research and development of next generation ultrahigh-speed transmission systems with a bit rate of 100 Gb/s or more per channel. Furthermore, intensive research and development of optical network systems that carry out all-optical routing of optical signals have been conducted. It is essential for a high-speed transmission system to evaluate the optical system and devices using high-speed optical random pattern pulse train, and for an optical network system to generate a high-speed label pulse train for optical packets.
FIG. 1 shows a conventional optical pulse pattern generator used for the foregoing purposes. In the optical pulse pattern generator shown in FIG. 1, an optical pulse train with a period T from an optical pulse source 1 is supplied to an input 2, and is split by an optical splitter 3. Then, individual optical pulses pass through optical waveguides 4-1 to 4-N, where N is an integer greater than one, and are led to optical switches 5-1 to 5-N. The optical pulses pass through only optical switches in a bar state among the optical switches 5-1 to 5-N to be led to the delay lines 6-1 to 6-N, and are coupled by optical combiners 7 to be output from an output 8. In this case, if length differences of the delay lines 6-1 to 6-N increase step by step by an amount of cT/(nN) in this order, an optical random pattern pulse train with a period T and a sequence length N is generated, where c is the light speed in the vacuum, and n is the group refractive index of the delay lines. The optical random pattern pulse train corresponds to the bar state (1) or a cross state (0) in each of optical switches.
Therefore to generate the optical random pattern pulse train with the period T and sequence length N, the foregoing conventional method must include N optical switches and N delay lines, thereby complicating the configuration because of an increase in the number of components and the size thereof. In addition, it requires 1xc3x97N optical splitter 3 and Nxc3x971 optical combiner 7, thereby increasing the loss.
For example, xe2x80x9cLarge-capacity WDM packet switchingxe2x80x9d K. Habara et al., Springer Photonic Networks (G. Prati Ed.), 1997 discloses in pp.285 to 299 a method of repeatedly launching optical pulses onto an optical device composed of an optical combiner and splitter, a delay line array and an optical switch array. However, it requires the same number of the delay lines and switches as the sequence length needed. Accordingly, its size increases and its configuration becomes complicated at a typical sequence length. In addition, since the number of the input ports of the combiner and that of the output ports of the splitter must also be equal to the sequence length, its loss increases with an increase in the sequence length.
Furthermore, R. J. S. Pedersen, B. F. Jorgensen, M. Nissov and He Yongqi, xe2x80x9c10 Gbit/s repeaterless transmission over 250 km standard fibrexe2x80x9d ELECTRONICS LETTERS, 7th Nov. 1996, Vol. 32, No. 23 discloses in pp.2155 to 2156, a method of modulating CW light by driving an optical modulator by an electric pulse pattern. However, it is difficult for it to generate a pulse pattern beyond 40 Gb/s because of the limit of the operation speed of the pulse pattern generator in an electrical region.
Furthermore, U.S. Pat. No. 5,208,705 discloses a method of utilizing a feedback shift register composed of an optical exclusive OR circuit based on nonlinear optical effect in combination with an optical fiber fixed delay lines. However, since it must use two types of optical pulses (clock pulse and control pulse), its configuration becomes large and complicated. In addition, using the nonlinear optical effect imposes some conditions on the operable optical pulse intensity, and makes its operation unstable. Furthermore, using optical fiber fixed delay lines makes it difficult to adjust the delay line length accurately, and to vary the pulse pattern, pulse period or bit rate.
As described above, no effective high-speed optical pulse train generating means are reported up to now. Consequently, implementing a small, stable all-optical pulse pattern generator that is not governed by the speed of electric components has been expected.
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide an optical pulse pattern generator with a simple configuration and low loss.
According to a first aspect of the present invention, there is provided an optical pulse pattern generator comprising: an optical pulse source for generating an optical pulse; an optical combiner and splitter having two inputs and two outputs, a first input of the two inputs of which is connected to the optical pulse source; a variable optical delay line circuit having two inputs and two outputs and including a plurality of cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers each of which has two inputs and two outputs, a first output of one of the cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers being connected to a first input of another of the cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers to form a cascade connection therebetween, and a first input of the variable optical delay line circuit being connected to a first output of the optical combiner and splitter; and one or more optical exclusive OR circuits, and inputs of the optical exclusive OR circuits being connected to second outputs of the cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers respectively, wherein a first output of the optical exclusive OR circuits is connected to a second input of the optical combiner and splitter.
Here, the optical exclusive OR circuits may be cascaded.
Each of the optical exclusive OR circuis may have two inputs and two outputs, and the number of the optical exclusive OR circuits may be less than the number of the cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers by one.
Each of the cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers may include at least one characteristic-variable asymmetrical Mach-Zehnder interferometer comprising: a first directional coupler with variable coupling ratio having two inputs and two outputs; and a second directional coupler with variable coupling ratio having two inputs and two outputs connected to the first directional coupler with variable coupling ratio through two optical waveguides with different lengths.
A first output of one of the characteristic-variable asymmetrical Mach-Zehnder interferometers may be connected to a first input of another of the characteristic-variable asymmetrical Mach-Zehnder interferometers to form a cascade connection of the two as each of the cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers.
One of the first and second directional couplers with variable coupling ratio may be used in common by two of the characteristic-variable asymmetrical Mach-Zehnder interferometers.
The optical pulse pattern generator may further comprise at least one optical amplifier at a position on a light path.
The optical combiner and splitter may consist of a symmetrical Mach-Zehnder interferometer.
Each of the optical exclusive OR circuits may consist of a symmetrical Mach-Zehnder interferometer.
The optical pulse pattern generator may further comprise an optical gating device connected to an output of one of the variable optical delay line circuit, the optical combiner and splitter and the optical exclusive OR circuits.
According to a second aspect of the present invention, there is provided an optical pulse pattern generator comprising: an optical pulse source for generating an optical pulse; an optical combiner and splitter connected to an output of the optical pulse source; a variable optical delay line circuit connected to the optical combiner and splitter; and one or more optical exclusive OR circuits connected to the variable optical delay line circuit at intermediate stages and a final stage of the variable optical delay line circuit, wherein one output of the optical exclusive OR circuits is connected to the optical combiner and splitter.
According to a third aspect of the present invention, there is provided an optical pulse pattern generating method of generating an optical random pattern pulse train from an optical pulse, the optical pulse pattern generating method comprising the steps of: launching an optical pulse produced by an optical pulse source to a variable optical delay line circuit via an optical combiner and splitter, the variable optical delay line circuit including a plurality of cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers; supplying an optical pulse output from each of the cascade-connected characteristic-variable asymmetrical Mach-Zehnder interferometers to the optical combiner and splitter through one or more optical exclusive OR circuits; and producing a random pulse train from one of the variable optical delay line circuit, the optical combiner and splitter and the optical exclusive OR circuits by using the optical pulse supplied to the optical combiner and splitter.
According to the present invention, the output of the optical pulse source is connected to the variable optical delay line circuit via the optical combiner and splitter. The variable optical delay line circuit feeds back from its intermediate or final stage the optical pulse to the optical combiner and splitter via the optical exclusive OR circuits. The configuration can implement a linear feedback shift register in the optical region. Adjusting the length of the variable optical delay line circuit makes it possible to generate optical random pattern pulse trains with various periods and patterns, thereby being able to realize the optical pulse pattern generator.
Furthermore, the optical pulse pattern generator in accordance with the present invention, by applying the variable linear feedback shift register configuration in the optical region, can generate the optical pulse patterns with various periods and patterns with a small size, low loss, integrated configuration without using a high-speed intensity modulator.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.